The present invention is based on Japanese Patent Applications No. 2000-033684 and No. 2000-063974 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a hollow rack shaft and, more particularly, to a hollow rack shaft for a steering system used in automobiles.
2. Description of Related Art
As the steering system for automobiles, a rack and pinion steering system is most frequently adopted. According to this rack and pinion steering system, when a steering wheel is turned by a driver, the rotation of this steering wheel is transmitted to a pinion. The rotation of the pinion is then transmitted to a rack shaft in mesh with the pinion, with the rotary motion being converted into a lateral (in the longitudinal direction of the rack shaft) motion. Because the rack shaft is connected to a steering rod, the direction of the front wheels is changed by the lateral motion of the rack shaft. The steering mechanism of this type is a known art and therefore will not be further explained.
The conventional rack shaft has been manufactured by gear cutting on a rod-like material. Recently, however, there has been the tendency for a rack shaft to be light-weightized in order to improve automotive fuel efficiency. In Japanese Patent Laid-Open Nos. Hei 06-246379, Sho 58-218339, and Hei 11-180318, methods of manufacturing the rack shaft have been disclosed.
According to the method of manufacturing the rack shaft disclosed in Japanese Patent Laid-Open No. Hei 6-246379, a tubular workpiece is inserted into a first split die, and the tublar workpiece is pressed by a primary press-forming process. Then the primary formed material is inserted into a second split die having the teeth on a part of the inner surface correspondingly to the rack teeth. Finally a semi-circular mandrel is pressed in from one end to thereby form the rack teeth.
The method of manufacturing the rack shaft disclosed in Japanese Patent Publication No. Sho 58-218339 involves inserting a mandrel into the tubular workpiece, and under the state that the tubular workpiece is surrounded by a stationary die, a second die having teeth corresponding to the rack teeth is pressed to the outside of the tubular workpiece, to thereby form rack teeth.
The method of manufacturing disclosed in Japanese Patent Laid-Open No. Hei 11-180318 is a technique invented by Okubo, one of inventors of the present invention. According to this method of manufacturing, an approximately rectangular plate is bent into a gutter shape along a longitudinal center. The gutter-shaped member has a flat bottom formed at the central part, and on both ends is provided a semi-circular shape with leg portions. On the flat bottom-shaped central part the rack teeth are formed through a plastic working process. Thereafter, the leg portions of the central part and the leg portions of both ends are bent into the form of a tube to thereby form a rack shaft.
In the technique of the first two patent documents described above, the tubular workpiece is used as a raw material, while in the latter technique a plate is used in place of the tubular workpiece. Therefore, not only is the invention advantageous in material cost, transportation cost and storage cost, but it has the merit which can achieve a substantial weight reduction effect without adding material to a portion other than the rack teeth in order to secure the thickness of the bottom section of the rack teeth.
The rack shaft is required to be compact and to withstand a great deal of load. This requirement is achievable by increasing the width of the rack teeth without increasing the whole body of the rack shaft.
In prior art methods, however, a tubular workpiece is used. Therefore, it is substantially impossible to manufacture the rack shaft in such a manner that the tooth width will exceed the size of the chord which crosses the outer periphery of the tubular workpiece (tube). That is, in the method disclosed in Japanese Patent Laid-Open No. Hei 6-246379, when the tubular workpiece is forced into the die corresponding to the rack teeth by the use of a mandrel, a small-diameter mandrel is pressed into a narrow tubular workpiece. Therefore, the diameter of the mandrel is limited in size and accordingly a rack having a large tooth width cannot be formed.
In the method disclosed in Japanese Laid Open Publication No. Sho 58-218339 (Examined Japanese Patent Publication No. Hei 4-28582), the die having teeth corresponding to the rack teeth is pressed on the tube to thereby forge the rack teeth. At this time, since the workpiece is upset in the direction of the center axis, it is almost impossible to increase the tooth width.
These two methods of manufacture have such a drawback that the inner side (the inner surface of the hollow section) of the rack teeth is flat, and therefore the tooth bottom section will decrease in thickness and strength compared with the other portion. Besides, these methods also have the drawback that it becomes necessary to add excess material to the other portion in case the metal thickness of the tooth bottom is to be assured, as a result a sufficient weight reducing effect cannot be obtained.
The above-described problem can be solved to some extent by the method of manufacture disclosed in Japanese Patent Laid-Open No. Hei 11-180318 described above, which, however, is still insufficient for the following reason.
Referring to FIGS. 1-5, in the latter method of manufacturing the hollow rack shaft, a gutter-shaped workpiece 103 (FIGS. 3a and 3b), having a flat central portion and a semi-circular portion with legs on both ends, is used. A rack portion 101 (FIG. 4a) is formed in the flat bottom section of the workpiece 103. To form the rack portion 101, an upper die 151, having a complementary shape (the tooth 106) of the rack tooth form in the lower part, and a lower die 152, having the inner side shape (the tooth 108) of the rack tooth form in the upper part, are used. Between these dies, the flat bottom section of the workpiece 103 is placed (FIGS. 3a and 3b). The workpiece is then press-formed by letting the upper die 151 down. On the sides of the upper die 151 and the lower die 152, pressure plates 153, 154, 155, and 156, to support workpiece 103 from the sides, are provided, as shown in FIG. 3b. 
As shown in FIG. 2a and FIG. 2b, the width B of the lower surface 107 of the tooth bottom section of the rack 101 in the hollow section 102 of the hollow rack shaft is the length of the chord geometrically determined by the radius R of the hollow section 102, tooth bottom position X, and plate thickness t of the rack tooth bottom. In a conventional rack and pinion steering system, the width A of a portion 108 to form the teeth of the lower die 152 has been designed to be the same size as the width B. In this type, the position X of the booth bottom requires to be more than a given amount in order to maintain the bending strength of the hollow rack shaft 100; therefore it is impossible to lower the position X (or X-t) to the vicinity of the center of the hollow portion 102. Therefore the width B must be made substantially smaller than the diameter (2R) of the hollow portion 102, and accordingly the rack tooth width could not be increased.
Furthermore, it has been understood that when the lower die 152 stated above is used, the effective tooth width C of the rack teeth 105 (the tooth width in the pitch surface of the rack teeth 105) is worked narrower than intended. A reason for the above-described drawback will be explained below. FIG. 5 is a view for explaining where the material of the workpiece 103 in the region of the flat bottom section before the forming of the rack teeth 105 transfers after the forming of the rack teeth 105. At the center of the rack teeth 105, the material within the region A7 has moved to the region a7, while the material within the region A8, to the region a8. In the meantime, near the end of the rack teeth 105 the material within the regions A1 to A6 has moved to the regions al to a6 respectively. The material that has moved as far as the vicinity of the end of the rack teeth 105 during the forming of the rack teeth 105 is stretched much wider than the material at the central part of the rack teeth 105, being spread to a wide area. That is, because the rack teeth 105 are formed in the flat bottom section, the plate thickness of the workpiece 103 in the vicinity of the end of the rack teeth 105 is decreased more than that in the central part of the rack teeth 105, resulting in a decreased effective tooth width C as indicated by the enclosure E of FIG. 4a (a sectional view taken along line IIIxe2x80x94III in FIG. 4a), and in FIG. 5. The decrease in the effective tooth width C will increase a load on the tooth face, resulting in a fracture of the rack teeth 105 or a shortened life of the hollow rack shaft 100.
It may be proposed to increase a pressure to be applied to the upper die 151 and the lower die 152, to thereby allow the flow of the material from the central part to the vicinity of the end of the rack teeth 105. This method, however, will exert a great deal of load to the teeth 108 of the dies 151 and 152, substantially, therefore, it is impossible to adopt this method. In case this method is adopted, there will arise the problem that the upper die 151 and the lower die 152 will considerably decrease in life.
Furthermore, when the decrease in the effective tooth width C of a product (the hollow rack shaft) is accepted, there will occur such a shortcoming that the user will have some doubts about the reliability of the product even if the product has none of such problems as deteriorated tooth face accuracy and function of a gear transmission, a fracture of the rack teeth 105, and shortened life of the hollow rack shaft 100.
According to the hollow rack shaft and the method of manufacturing the hollow rack shaft of a first embodiment of the present invention, the hollow rack shaft is formed from a plate in order to obtain a sufficient tooth width even in the vicinity of the end of the rack teeth, thereby providing the rack teeth with a sufficiently wide tooth width and consequently reducing a load per unit surface area of each rack tooth face. Therefore, it is possible to prevent damage to the rack teeth, thereby enabling the prolonging of the life of the rack shaft. Furthermore, because of the provision of the sufficiently large rack tooth width, the user will have no doubts about the reliability of the rack shaft product.
Furthermore, in the hollow rack shaft of the first embodiment, the width of rack teeth on the hollow inner surface side at the cross section at right angles to the axis of the hollow rack shaft is wider than the width geometrically determined by such dimensions as the inside diameter of the hollow rack shaft, the position of the rack tooth bottom, and the plate thickness of the rack tooth bottom.
Furthermore, each tooth of the hollow rack shaft of the first embodiment is formed to have a greater formation at both ends in the direction of the tooth trace than the central portion.
Furthermore, when each tooth of the hollow rack shaft of the first embodiment is formed, the flat bottom section of the workpiece is formed by a press-forming process by the use of a die which is wider than the width of the flat bottom section.
The tooth of the die is recessed at the central portion and provided with projections on both end portions, thereby providing a greater formation at both end portions of the tooth to be formed than the central portion.
Furthermore, according to another method of manufacturing the hollow rack shaft in accordance with a second embodiment of the present invention, an approximately rectangular plate which has been shaped wide at the rack tooth portion formed in post-process is bent lengthwise into a form having a gutter-shaped cross sectional form. At this time, at its center portion the bent plate has a flat bottom section on which the rack teeth are to be formed and a pair of mutually facing leg portions, and each of the end portions also has the semi-circular bottom section with a pair of mutually facing leg portions. The mutually facing leg portions at the center have a larger spacing than the spacing between the mutually facing legs in the vicinity of both end portions. Rack teeth are formed on the flat bottom section through the plastic flow of the flat bottom section and a part of the material of the pair of leg portions of the flat bottom section. The bent plate on which the rack teeth have been formed is bent further at the leg portions until their edges are butted, thus forming a hollow tube-like rack shaft.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of preferred embodiments of the present invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate examples of the present invention Such examples, however, are not exhaustive of various embodiments of the present invention, and therefore reference will be made to the claims which follow by the description for determining the scope of the present invention.